Peter Robrish

Real-time imaging using a 2.8 THz quantum cascade laser and uncooled infrared microbolometer camera

Real-time imaging in the terahertz (THz) spectral range was achieved using a milliwatt-scale, 2.8 THz quantum cascade laser and an uncooled, 160 x 120 pixel microbolometer camera modified with Picarin optics. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was estimated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. Despite the appearance of fringe patterns produced by multiple diffraction effects, single-frame and extended video imaging of obscured objects show high-contrast differentiation between metallic and plastic materials, supporting the viability of this imaging approach for use in future security screening applications.

Imaging at 3.4 THz with a quantum-cascade laser

We have assembled a single-frequency imaging system at 3.4 THz with a quantum-cascade laser. Images of electronic and biological applications are demonstrated. We operate the laser with a peak output power of 2.5 mW at a 7% duty cycle and a 22 K operating temperature. The minimum spot size is 340 microm. The signal is detected with a single-element deuterated triglycine sulfate detector, and images are captured by scanning of the sample.

Design and performance of a reconfigurable liquid-crystal-based optical add/drop multiplexer

The authors describe a reconfigurable optical add-drop multiplexer designed for 50-GHz channel spacing over either the C- or L-bands. The system may also function as a variable optical attenuator. The design features a reflective liquid-crystal modulator, a compact free-space spectrometer, polarization diversity, and fine-scale attenuation control.

Detection of 3.4 THz radiation from a quantum cascade laser using a microbolometer infrared camera

The THz wavelengths cover the frequency range of 0.1-10 THz or 30-3000 &mgr;m wavelength band. Currently, detection of THz radiation is carried out using either antenna-coupled semiconductor detectors or superconducting bolometers. Imaging of objects using these detection schemes requires complex scanning mechanisms which limits the applications involving real time imaging. For imaging applications it is desirable to employ focal plane arrays (FPAs) which leads to more compact systems. The FPAs based on photon detectors commonly used in infrared require cooling which becomes stringent as the detection extends to THz wavelengths. On the other hand, microbolometer FPAs using thermal detectors based on temperature change due to infrared absorption have a broad wavelength response and can be operated at room temperature. The advances of microbolometer technology allow real time imaging in the 7-13 &mgr;m wavelength range with relatively high sensitivity. However, their ability to detect THz radiation is relatively unknown. In this paper, imaging of a 3.4 THz (88 &mgr;m) laser beam using an uncooled microbolometer camera is described.

Group delay reference artifact based on molecular gas absorption

We propose a novel group delay reference for optical communication systems based on a molecular gas cell absorption characteristic. A 1.53 /spl mu/m C/sub 2/H/sub 2/ acetylene gas absorption cell commonly used as a wavelength reference is shown to have an important dispersive nature that forms the basis of a novel group delay reference. Group delay measurements are compared to theoretical predictions based on the amplitude response of the molecular gas cell. The formulated analytical approach is applicable to an arbitrary absorption spectra described by a Lorentzian function.

Dual-wavelength THz imaging with quantum cascade lasers

We have assembled an imaging system using quantum cascade lasers at frequencies of both 3.4 and 2.3 THz. Images at the two frequencies and the resulting absorption coefficients are compared. We demonstrate imaging in both reflection and transmission. The lasers are operated in a closed-cycle refrigerator and we use a peak output power of >2.5 mW at 7-10% duty cycle and 22-40 K operating temperature. The focal spot size is approximately 300 microns for both lasers and is not diffraction-limited. The signal is detected with a single-element DTGS detector, and images are captured by scanning the sample. Applications enabled by longer wavelengths are demonstrated, as well as the determination of chemical information through imaging at two wavelengths.

Optimization of a 3.6-THz quantum cascade laser for real-time imaging with a microbolometer focal plane array

Real-time imaging in the terahertz (THz) spectral range was achieved using a 3.6-THz quantum cascade laser (QCL) and an uncooled, 160×120 pixel microbolometer camera fitted with a picarin lens. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was calculated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. After evaluating the effects of various operating parameters on laser performance, the QCL found to perform optimally at 1.9 A in pulsed mode with a 300 kHz repetition rate and 10-20% duty cycle; average output power was approximately 1 mW. Under this scheme, a series of metallic objects were imaged while wrapped in various obscurants. Single-frame and extended video recordings demonstrate strong contrast between metallic materials and those of plastic, cloth, and paper - supporting the viability of this imaging technology in security screening applications. Thermal effects arising from Joule heating of the laser were found to be the dominant issue affecting output power and image quality; these effects were mitigated by limiting laser pulse widths to 670 ns and operating the system under closed-cycle refrigeration at a temperature of 10 K.